Integrated circuits are now used in almost every type of electronic product ranging from toys to massive computers. These integrated circuits are all generally made by a photolithographic process, which involves manufacturing a template containing patterns of the electrical circuit as transparent and opaque areas. The patterned template is referred to as a “reticle” or “mask”.
A radiation source, such as a light, is used to copy or “pattern” multiple images of the mask onto a photosensitive material, such as a photoresist, on the surface of a silicon wafer. Once features are patterned on the photoresist, further processing is performed to form various structures on the silicon wafer. The completed wafer is then cut (or “diced”) to form the individual integrated circuits.
Engineers typically use computer aided design (“CAD”) to create a schematic design of the mask. One technique, Levenson Phase-Shifting, also known as Alternating Aperture Phase-Shifting, is used to create small features on integrated circuits. Such small features are generated by a pair of areas in the mask called shifters separated by an opaque region.
The opaque region, typically made of chrome, does not allow radiation such as light to pass through. However, the shifters allow light to pass through and change the phase of the light. Two shifters can be used to shine light on the same region of a photoresist. When the light passing through one of the shifters is out of phase with the light passing through the other shifter, a feature is created on the photoresist that is narrower than the opaque region separating the shifters. By reducing the size of the opaque region, and thus the distance between the two shifters, very small features can be created on the photoresist. The width of the feature can be considerably less than could be produced by the same optical system without phase shifting.
When light from one shifter is 180° out of phase and overlaps with the light from the other shifter, destructive interference occurs and the light from the two shifters cancels. Controlling the phase of the light passing through the mask can be extended to the point where all of the opaque regions are created by destructive interference as opposed to blocking the light with chrome. A chromeless phase-shifting mask transmits 100% of the light and is used in a technique called phase edge chromeless off axis lithography (“PCO”), also known as chromeless phase lithography (“CPL”).
However, the use of CPL can cause artifacts to appear. Artifacts are unwanted features in the photoresist created by areas of secondary light intensity. One type of artifact is a side lobe. Side lobes typically appear as small halos in the photoresist.
One technique used to prevent side lobes is the use of a chrome patch. When properly placed on the mask, chrome patches block light of secondary intensity and prevent side lobes from forming in the photoresist.
Unfortunately, shifters can limit the placement and dimensions of chrome patches. Furthermore, as photolithographic technology continues to advance and mask designs continue to shrink, the risks of chrome lifting increase. Chrome lifting is the loss of the chrome patch, or portions of the patch, during processes such as mask cleaning.
What is needed, therefore, are improved methods and configurations for preventing side lobes from forming while decreasing the risk of chrome lifting.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.